Method and apparatus for avoiding nitric oxide-containing deposits in gas decomposing plants



1963 F. ROTTMAYR 3,

METHOD AND APPARATUS FOR AVOIDING NITRIC OXIDE- CONTAINING DEPOSITS IN GAS DECOMPOSING PLANTS" Filed Dec. 2, 1960 3,191,262 METHOD AND APPARATUS FOR AVGHDENG NITRIC @XlDF-CQNTAIINH IG DEPQSITS IN GAS DECQMPOSlNG PLANTS Friedrich Rottmayr', Pullaeh, near Munich, Germany, assign'or to Gesellschaft fur Lindes Eisrnaschrnen Aktiengesellsehatt, Hollriegelskreuth, near Munich, Germany, a corporation of Germany Filed Dec. 2, 1960, Ser. No. 73,291 Claims priority, application Germany Dec. 5, 1959 4 Claims. (6i. 62-23) This invention concerns a method of and an apparatus useful for avoiding nitric oxide-containing depositspa rticularly, NO-resins such as nitrosites and nitrosates-m gas decomposing plants, particularly coke oven gas decomposing plants.

It is known to free coke oven gas of ammonia, hydrogen sulfide, carbon dioxide and the like by washing, and to decompose the thus-prified gas in a low temperature plant. The gas is first cooled; then by further cooling is freed of hydrocarbons and carbon monoxide, so that pure hydrogen and a hydrogen-nitrogen mixture are finally obtained. To this end there are provided several counterflow heat-exchangers which are arranged at different temperature levels. The constituents which are, separated in liquid form in these heat-exchangers are combined at the bottom end of the heat-exchangers in separators and then conducted to an evaporator.

Usually the first counterflow device, whose lowest temperature is about -110 C., is called the hot leg: in it are separated preferably the hydrocarbons which contain three or more carbon atoms per molecule. The following heat-exchanger, which cools down to about 145 C., is called the cold leg: in it are separated mainly the compounds with one and two carbon atoms per molecule, like methane, ethane, ethylene. In a final piece of equipment, the so-called methane vapor-counter-flow heat-exchanger, which cools down to about 180 C., the bulk of the methane and carbon monoxide are condensed. The remaining hydrogen and hydrogen-nitrogen mixture, respectively, is subsequently freed, in a wash column, from the last traces of carbon monoxide, methane and oxygen.

The coke oven gas also is known to contain traces of nitric oxide (NO) which, though it is present only in very small amounts, e.g., several tenths of one part per million (p.p.m.), has a very disturbing effect in the decomposing apparatus because it is oxidized by the oxygen contained in the gas to N and N 0 and these compounds react with the higher olefins, particularly with the diole fins, forming nitrosites and nitrosates which are deposited in certain parts of the apparatus and constitute an explosion hazard. Explosions of such compounds lead frequently to the destruction of the evaporator for the hydrocarbons separated in liquid form.

An object of the present invention is, therefore, to improve the general process in such manner as to avoid as far as possible the formation of these resins and thus explosion hazards engendered by the same.

This object is realized, according to the invention, by a method for avoiding nitric oxide-containing deposits in gas decomposing plants, particularly coke oven gas decomposing plants, where from pre-purified gases which contain only 1ow-boiling substances these constituents are separated by condensation in at least two series-connected heat-exchanges arranged at different temperature levels, which method is characterized in that the liquids separated from each heat-exchanger are collected separately and treated separately, preferably evaporated.

Applicant has found that only a small portion, say,

about 5 to of the nitric oxide contained in the gas is oxidized in the hot leg to N0 and separated there,

United States Patent 0 3,191,262 Patented Aug. 20, 1963 while the bulk of the NO'content in separated in the fol lowing, colder, heat-exchangers, mostly in the cold leg, where about 50 to 70% of the existing nitric oxide is precipitated. On the other hand, the higher olefins-particularly, the diolefins-which mainly react with nitric dioxide, are preferentially separated (namely, more than in the hot leg.

According to the invention, then, by the separate elimination of the two fractions it is prevented that the two constituents (which are so reactive with each other) meet in liquid form and react with each other forming resins.

The liquid fractions withdrawn from the various heatexchangers are collected separately and are evaporated in separate evaporators, and the resulting gases are conducted to another processing plant, e.g., to an ethylene producthese liquids are conducted in known manner from the bottom to the top through the evaporators, it is possible that difiicultly vaporizable portions of the liquid, which contains in any case a great portion of N0 and of the resins, will accumulate on the bottom or in the lower parts of the evaporator and will lead to explosions there, though the probability of these accumulations is very low with separate evaporation.

According to another feature of the invention, then, the fractions accumulated in thetseparators are no longer conducted from the'bottom to the top through the respec tive evaporator but rather are conducted from the top to the bottom. The droplets, which have not completely evaporated and which contain relatively large amounts of N0 and resins, are removed from the evaporator with the downstream and are evaporated, if not sooner, during the subsequent heating of the gas outside of the decomposing apparatus.

The invention can be supplemented in an advantageous manner in this way that an additional separator is provided in the connecting lines between two heat-exchangers used for the separation, particularly in the connecting line between the cold leg and the methane evaporation counterfiow heat-exchanger, in which liquid droplets that have been carried along are deposited. This prevents these droplets, which contain particularly large amounts of NO, from getting into the following colder heat-exchangers -Where they could and would cause the separation of resins. The liquid accumulating in these separators is fed to the evaporators corresponding to its composition.

An apparatus for carrying out the method according to the invention is represented schematically and by Way of example in the attached drawing.

The coke oven gas, freed from benzol, ammonia, hydrogen sulfide and carbon dioxide, and cooled to a temperature of about 40 C., is fed through conduit 11 to a heat-exchanger 1, the so-called hot leg. Here the gas is cooled by indirect heat-exchange with counterflowing methane (conduit 12) and hydrogen (conduit 13), to about --l10 C. Here the hydrocarbons having three and more carbon atoms per molecule are mainly separated and are fed to a separator 6 through conduit 14. The coke oven gas flows, then, through conduit 15 to a heatexchanger 2, which is generally called the cold leg. Here the gas is cooled to about C. by indirect heatexchange with the gaseous hydrogen, methane and carbon monoxide flowing through conduits 13, 12 and 16, with hydrocarbons having one and two carbon atoms per molecule being mainly separated. These [fractions are fed through conduit 17 to a separator 8, whilst the gas flows through conduit 18 to a separator 3 and, from there,

through conduit 19 to a so-called methane evaporation counter-flow heat-exchanger 4 for the separation of liquid droplets that have been carried along. In heat exchanger '4 the gas is again cooled to about 180 C. by heat-exchange with hydrogen, methane and carbon monoxide, whereby the methane is liquefied and conducted through conduit 20 to the so-called nitrogen evaporator.

The succeeding steps, not being a part of the present invention, may very briefly be describedas follows: The liquid methane-carbon monoxide mixture accumulating in the base of the nitrogen evaporator is expanded through a valve 22, is evaporated and heated in heat-exchangers 4 and 2 and leaves the low temperature zone through conduit 16.

The liquids collected in separators 6 and 8 are evaporated in evaporators 7 and 9 by indirect heat-exchange with high pressure nitrogen which is supplied, at 23, at a temperature of about 45 C. and a pressure of about 100 to 200 atm. abs. The evaporation products leave the decomposing apparatus through conduits 24 and 25 and can be combined, after additional heating in counterflow precoolers (not represented), and conducted to an additional processing plant, preferably an ethylene production plant. The nitrogen cooled in heat-exchangers 7 and 9 flows through conduit 26 and pipe coils 27, arranged in the so-called nitrogen evaporator, and is expanded through a valve 28 into a wash column 5.

The hydrogen-nitrogen-ca-rbon monoxide mixture issuing from the head of nitrogen evaporator 10 is conducted through conduit 30 to column 5. Here the last traces of carbon monoxide, oxygen and methane are washed out with washing nitrogen. The hydrogen flows through a conduit 13 from the head of the column, while the liquid nitrogen-carbon monoxide mixture accumulates in the base of the column. This lattermixture 'is expanded through conduit 31, containing expansion valve 32, into part 29 of a nitrogen evaporator 10 and is evaporated there; it is withdrawn in gaseous form, through conduit 16, and heated again in the heat-exchangers 4 and 2. The liquid accumulating in separator 3 is also conducted through conduit 21 to evaporator 9,

I claim:

1. Method for avoiding nitric oxide-containing deposits in gas decomposing plants, particularly coke oven gas decomposing plants for the production of hydrogen gas where from the pre-purified gases containing only lowboiling constituents including aliphatic hydrocarbons and carbon monoxide these constituents are separated by condensation in at least two series-connected heat-exchangers arranged at different temperature levels, characterized in that the separated liquids separately collected from each heat-exchanger are conducted in apparatus units separate from said heat-exchangers to be evaporated by indirect heat-exchange with high pressure nitrogen flowing in countercurrent to them, nitrogen being used in a known way after expansion as a liquid washing medium in a wash columnin order to wash out the last traces of undesired impurities from the hydrogen gas to be produced.

2. Method according to claim 1, characterized in that the separated liquids are conducted through separate evaporators from the top to the bottom for evaporation.

3. Apparatus for avoiding nitric oxide-containing deposits in gas decomposing plants for the production of hydrogen gas, said apparatus including at least two seriesconnected heat-exchangers arranged at different temperature levels, each or said heat-exchangers being equipped at its bottom with a separator for condensed liquid, characterized in that each separator is connected by a separate line with the upper end of one evaporator which is separate from said heat exchangers.

4. Apparatus according to claim 3, further characterized in that a separator is provided in at least one connecting conduit between two heat-exchangers, said separator being connected with the upper end of the separate evaporator for the condensed liquid coming from the preceding of said two heat-exchangers.

References (Iited in the file of this patent UNITED STATES PATENTS 1,830,610 Linde Nov. 3, 1931 1,865,135 Pollitzer et a1. June 28, 1932 1,913,805 Hausen June 13, 1933 2,122,238 Pollitzer June 28, 1938 2,591,658 Haringhuizen Apr. 1, 1952 

1. METHOD FOR AVOIDING NITRIC OXIDE-CONTAINING DEPOSITS IN GAS DECOMPOSING PLANTS, PARTICULARLY COKE OVEN GAS DECOMPOSING PLANTS FOR THE PRODUCTION OF HYDROGEN GAS WHERE FROM THE PRE-PURIFIED GASES CONTAINING ONLY LOWBOILING CONSTITUENTS INCLUDING ALIPHATIC HYDROCARBONS AND CARBON MONOXIDE THESE CONSTITUENTS ARE SEPARATED BY CONDENSATION IN AT LEAST TOW SERIES-CONNECTED HEAT-EXCHANGERS ARRANGED AT DIFFERENT TEMPERATURE LEVEL, CHARACTERIZED IN THAT THE SEPARATED LIQUIDS SEPARATELY COLLECTED FROM EACH HEAT-EXCHANGER ARE CONDUCTED IN APPARATUS UNITS SEPARATE FROM SAID HEAT-EXCHANGERS TO BE EVAPORATED BY INDIRECT HEAT-EXCHANGER WITH HIGH PRESSURE NITROGEN FLOWING IN COUNTERCURRENT TO THEM, NITROGEN BEING USED IN A KNOW WAY AFTER EXPANSION AS A LIQUID WASHING MEDIUM IN A WASH COLUMN IN ORDER TO WASH OUT THE LAST TRACES OF UNDESIRED IMPURITIES FROM THE HYDROGEN GAS TO BE PRODUCED. 